A pattern aligning device for a sewing machine, is known from U.S. Pat. No. 4,757,773 (based on West German Offenlegungsschrift No. 3,738,893). Using the prior-art device, it is possible to sew together, according to a pattern, two fabric layers having the same surface structure or the same pattern. In the case of, e.g., a check pattern, first pattern lines of one of the fabric layers extending at right angles to the seam line are to be aligned with the corresponding pattern lines of the other fabric layer, and second pattern lines of the other fabric layer extending in parallel to the seam line are to have the same lateral distance from the seam line as the corresponding pattern lines of the other fabric layer.
As a prerequisite for aligning pattern lines extending in parallel to the seam line, one of the fabric layers first must be aligned to a predetermined edge distance from the stitch formation site. An edge sensor is provided for this purpose, which determines the distance to the edge of the fabric layer. If the actual distance value deviates from the nominal distance value, the aligning means of the corresponding edge guiding device is actuated so that the deviation between the nominal value and the actual value is eliminated. The other fabric layer is now aligned to the correct pattern by means of the other aligning means of the edge guiding device on the basis of the data obtained for a transverse mismatch that may be present by the cross-correlation analysis.
The prior-art pattern aligning device is used mainly to produce the center back seam on jackets, because perfect appearance of this seam is a very important quality criterion for the evaluation of the overall quality of these garments. It is very important here, in the case of check patterns, for the transversely extending pattern lines of the left and right parts to be perfectly aligned with each other and for the longitudinally extending pattern lines to be located at exactly the same distance from the seam line. However, because of the greater width of the jacket in the shoulder area, pattern lines that are otherwise aligned in parallel to the seam line enclose an acute angle with the seam line in this area. In the case of certain patterns, in which the distance between longitudinally extending pattern lines exceeds the width of the surface sensor, it may now occur in the shoulder area that the obliquely extending longitudinal lines move out of the field of measurement of the surface sensor, so that no longitudinal lines can be detected for a certain length of time, and consequently no signal indicating the transverse distance of the longitudinal lines can be obtained. Even though it would now be possible to use surface sensors of correspondingly large size in order to avoid this situation, such surface sensors are extremely expensive.